(1) Field of the Invention
The invention relates to the general field of etching by non chemical means, particularly RF sputter-etching.
(2) Description of the Prior Art
RF (radio frequency) sputter-etching is a universal etching technique, although the actual rate at which material gets removed does vary from one material to another. DC (direct current) sputter-etching is similar, but is limited to electrically conductive material, which RF sputter-etching is not. The basic setup is to provide two electrodes--a substrate holder and a sputtering shield, immerse them in gas at low pressure and apply an RF voltage between them.
FIG. 1 shows an exploded isometric view of part of a standard sputter-etcher of this type, in this case Electrotech's model MS6210 which was used in the development and reduction to practice of the present invention. During use, silicone oil cooled substrate holder 1 is positioned just inside the open lower portion of sputtering shield 2. Air cooled lid 4 is attached to 2 along with vacuum tight O-ring seal 3. The various attachments labelled as 5 include means for admitting the sputtering gas through lid 4 and controlling its pressure, as well as means for applying the RF power.
FIG. 2 is a schematic cross-section of FIG. 1. Gas baffle 11 serves the important purpose of dispersing the stream of incoming gas to reduce local variations in pressure within the sputtering chamber. It is customarily attached to shield 11 by standard round-headed screws, such as 21 and a space is allowed between it and the underside of 2. Incoming gas is forced to flow round to the sides of the baffle whose actual position within shield 2 can be seen in FIG. 2.
Not shown in FIGS. 1 and 2 is a vacuum chamber within which the sputter etcher resides during use. An appropriate sputtering gas (usually argon) is admitted into said vacuum chamber during use, its admission rate being adjusted so that some particular desired pressure level can be maintained. Said pressure is commonly about 6.times.10.sup.-3 torr. RF voltage is applied between the substrate holder and the sputtering shield, initiating an RF glow discharge, and material is removed from both electrodes at a rate that is inversely proportional to some power (between 1 and 4, depending on the exact geometry) of the ratio of their two areas. See, for example, Koenig in U.S. Pat. No. 3,661,761 May 9 1972. Since the area of the substrate holder is, by design, significantly less than that of the sputtering shield, the rate at which material will be removed from the substrate (assumed to be covering almost the entire substrate holder surface) will be substantially greater than the rate of material removal from the sputtering shield.
Sputter-etchers of the type illustrated in FIGS. 1 and 2 are intended for the sputter-etching of a single semiconductor wafer at a time. This allows for better control of the amount of material removed from different wafers than is possible in a batch system which processes many wafers at a time. However, in order to make the throughput of such a system economically attractive in a manufacturing environment, it is necessary that the time taken to etch a given wafer be kept as low as possible. This implies that significantly higher etch rates, and therefore significantly higher levels of RF power density, must be used relative to the batch methods. To avoid the need for very high RF voltages to achieve these high power densities (typically about 300 watts per wafer) these single wafer etchers operate at higher gas pressures (where plasma resistance is lower) than do the batch units.
This use of high pressure, and particularly high power, is known to have certain undesirable side effects, notably the non-uniform removal of material and the generation of fine dust particles, some of which find their way onto the surface that was sputter-etched. Any particulate matter of this type, if allowed to settle on the surface of an integrated circuit during the course of its manufacture, has a high probabilty of destroying said circuit, thus reducing the overall product yield.
A number of possible sources of non-uniformity of material removal and of particulate contamination are believed to exist. One of these is the effect of local fluctuations in gas pressure during sputtering. With improper baffle design the local density of the incoming gas during sputter etching can vary and therefore so will the plasma density. As a result, there will be small local variations in the sputter etch rate, leading to non-uniformity in the amount of material removed across the surface of wafer 25 in FIG. 2. Broken line 20 shows a profile of peak density for the incoming gas. Note that the low point of said profile is still some distance above the center of silicon wafer 25. This is believed to be a result of the relatively high gas flow rate associated with this baffle design.
The solution to these problems, which forms the subject matter of the present invention, involves the redesign of the gas inlet baffle 11 so that the afore-mentioned local fluctuations in gas density are much reduced.